The present invention relates to a cylinder liner and a cylinder block, which are used in an engine of a vehicle, and a method of manufacturing them.
A conventional cylinder block of an engine is shown in FIGS. 7 and 8. The cylinder block 1 includes: cylinder liners 2 made of cast iron; an aluminium block 3 enclosing the cylinder liners 2; and water paths 4, through which cooling water is introduced.
Generally, cold shut and scattered structures in aluminium occur defects in aluminium-casted products.
In the case of the cylinder block 1, rate of contraction of aluminium is very high when aluminium is cooled to solidify. On the other hand, the cylinder liners 2, which have been casted with cast iron, are not thermally influenced, so that great residual stress left in the aluminium portions.
In a portion "A" (see FIG. 8), which is between the adjacent cylinder liners 30, the aluminium contracts in directions of arrows. If there is an inner defect in the portion "A", the inner defect is apt to make a crack therein. When the aluminium is cooled and contacts, the residual stress, which is caused by the contraction of the aluminium, works to various portions. Especially, the stress works to the portion "A" and portions "B" when the cylinder liners 2 are machined or pistons slide in the cylinder liners, so that the cracks are apt to be formed in the portions "A" and "B".
To prevent forming the cracks, the portions "A" and "B" are made thicker, but this countermeasure makes the cylinder block 1 bigger and heavier.
No alloy is formed between the cast iron of the cylinder liners 2 and the aluminium of the alminium block 3, so the cylinder liners 2 and the aluminium block 3 cannot tightly adhered. There are many minute clearances there between. With the minute clearances, cooling efficiency of the cooling water must be lower.
To tightly adhere the cylinder liners 2 and the aluminium block 3, many ring projections are formed in outer circumferential faces of the cylinder liners 2.
The cylinder liner 2 having the ring projections is casted in a splittable molding unit, etc.
An example of the splittable molding unit is shown in FIG. 9.
The splittable molding unit includes a lower mold 10 and an upper mold 12. A core 14 is sandwitched between the molds 10 and 12, so that a casting space is formed between an outer circumferential face of the core 14 and inner faces of the molds 10 and 12. Molten metal is introduced into the casting space. Note that, symbol "P.L" indicates parting faces of the molds 10 and 12.
Another splittable molding unit is shown in FIG. 10.
The splittable molding unit includes a lower mold 10, which has an island section 10a, and an upper mold 12, which has an island section 12a. A casting space is formed between outer circumferential faces of the island sections 10a and 12a and an inner face of the lower mold 10. Molten metal is introduced into the casting space. Note that, symbol "P.L" indicates parting faces of the molds 10 and 12.
In the case of the splittable molding unit shown in FIG. 9, ring grooves, which correspond to the ring projections, are formed in the inner faces of the molds 10 and 12. The ring projections can be formed in the cylinder liner by employing the splittable molding unit, and the cylinder liner can be easily taken out from the molds.
However, many flashes are formed in an outer circumferential face of the cylinder liner along the parting faces "P.L". Namely, the flashes are formed between the ring projections, so it is difficult to perfectly remove the flashes. If the flashes are left, the residual stress works to the aluminium block, which encloses the cylinder liners, and cracks are apt to be formed in the aluminium block.
On the other hand, in the case of the splittable molding unit shown in FIG. 10, outer diameter of the cylinder liner is not fixed in the axial direction because angle for drafting the cylinder liner from the molds must be required. Therefore, the casted cylinder liner must be machined so as to have fixed outer diameter and form the ring projections.
By forming the ring projections in the cylinder liner, adhesion between the cylinder liner and the aluminium block is improved. However, the contraction rate of the aluminium in the circumferential direction of the cylinder liner is still great; the contraction rate of the aluminium in the axial direction of the cylinder liner is not so great. So, the residual stress still damages the aluminum block when the aluminium is cooled.